Look on the passenger side of the transmission housing. It is often positioned on the lower rear face of the rear housing.
Typically found on the passenger side (right side) of the transmission. Allison Transmission Serial Number Lookup
Several third-party websites (like "TransmissionDigest" or "PartsPro") offer lookup tools. Be very careful here. These databases are often outdated. If the site asks for $49.99 for a "full report," you are likely being scammed. Only use these for rough cross-references of filters, not for critical internal parts. Look on the passenger side of the transmission housing
Authorized dealers use software called Allison Doc . Allison Transmission Serial Number Lookup